Tropospheric propagation refers to the ability of radio energy to propagate farther than planned during periods of stable weather. In one propagation method, when a radio signal encounters an abrupt change in atmospheric density, often caused when cool damp air becomes trapped under warm dry air (known as a temperature inversion), the higher refractive index of the atmosphere will cause the signal shooting outwards to reflect back towards the ground. This effect is known as Tropo-refraction, and it can cause signals from one wireless telecommunications network market (“market”) to appear in an adjacent market, by reflecting off this “boundary layer” between the two air masses.
As shown in FIG. 1b, an illustration of tropospheric refraction is shown where a base station in one market (shown as Geo Region A) affects devices in a nearby market (shown as Geo Region B). The devices in Geo Region B encounter interference and noise from the RF transmission of the base station in Geo Region A. The distance between the two markets can be quite significant, upwards to 500 miles, but normally are found to be less than 100 miles. Although the term base station is used, the terms base station, e-Node B (eNB), and base transceiver station (BTS) may be referred to interchangeably. One of ordinary skill in the art knows that the base station, eNB, and BTS refer to various implementations in wireless telecommunication networks.
In FIG. 1c, another illustration of tropospheric refractions is shown. FIG. 1c is similar to FIG. 1b with the difference that FIG. 1c shows only two boundary layers of air. The cool damp air is closest to the ground with a layer of dry warm air existing over the cool damp air. As a result, a signal sent from Geo Region A reflects off the boundary between the cool damp air and dry warm air to affect devices (or user equipment) located in Geo Region B.
Tropospheric refraction is generally impactful to Frequency Division Duplex (FDD) markets through a mechanism of an eNB or BTS in a first market interfering with user equipment (UE) in a second market. This interference is the result of an increase in call failure rate (CFR) and call drop rate (CDR) without an increase in noise at the eNB or BTS. The interference my also impact time division duplex (TDD) markets where signals in one market impact user equipment in the second market. The interference may appear reciprocal due to the number of user equipment involved in each market, but this is not always the case. For example, a large network market may interfere with a smaller network market, but not vice versa.
Tropospheric ducting is a related effect that impacts frequencies up to about 8 GHz. Ducting occurs when two boundary layers form in the troposphere, and radio signals get trapped between them. Under these conditions, signals can propagate over 200 miles. This effect predominately occurs between base station locations with antennas that are either in, or can broadcast into, the “ducting layer.” Because tropospheric ducting can be a reciprocal effect, it is seen predominately in co-channel Time Division Duplex (TDD) operations. Tropospheric-related interference incidents tend to be short lived, lasting a few hours around sunrise and/or sunset, sometimes re-occurring daily throughout an entire season in reciprocal markets.
Tropospheric ducting is a BTS to BTS direct interference effect that is unique to co-channel TDD operation. The interference from tropospheric ducting may not always be reciprocal due to the number of stations (BTS) involved. For example, a network in Chicago would have a higher number of stations than a network in Grand Rapids, resulting in tropospheric ducting that is not reciprocal.
As shown in FIG. 1a, an illustration of tropospheric ducting is shown where a base station in one market (shown as Geo Region A) affects a base station in a nearby market (shown as Geo Region B). The RF transmission of each base station affects the other base station by causing interference and noise. As for tropospheric ducting, the distance between the two markets can also be quite significant.
The above two tropospheric effects (tropospheric ducting and tropospheric refraction) cause co-channel and other interferences. With increasing deployment in 2.5 GHz on Time-Division Long-Term Evolution (TD-LTE) technology as well as an increase in site densification, tropospheric propagation gains more relevance with RF propagating in one market being seen in another market using the same physical cell identifications (PCIs) and frequencies, leading to co-channel interferences and increased noise resulting in dropped calls. Radio waves from one market enter and travel significant distances within an atmospheric “duct” and can impact a customer's service in a distant market. The long distances that a signal can travel are based on the refractive index over the signal path and radio propagation conditions and coverage. Significant levels of interference for periods of time can disrupt radio communications links.
A solution is needed that can detect and identify tropospheric propagation. A solution is also needed that can reduce or remove tropospheric propagation when it occurs.